Since Sertoli cells of the rat populate the seminiferous tubules during perinatal life, and lose the ability to divide thereafter, they constitute a fixed, non-dividing population in the adult. Therefore, the perinatal period when Sertoli cells proliferate is presumably critical for normal adult testicular function. In these studies, the duration of this period of Sertoli cell proliferation will be determined by autoradiography following exposure to fetal and neonatal testes to 3H-thymidine, and correlated with 1) the ontogeny of FSH receptors on Sertoli cells in fetuses, 2) the potential of Sertoli cells in perinatal rats to aromatize androgens in vitro, and 3) the production of NADPH required for aromatization by Sertoli cells, as measured by quantitative cytochemistry. These data will form the basis for studies aimed at establishing the critical nature of the period of Sertoli cell proliferation. Division of these cells in neonates will be inhibited with cytosine arabinoside. Anti-serum to FSH and probably inhibin will by given to perinatal rats to reduce functional or absolute levels of FSH, respectively. Testicular function will be evaluated in adults treated, as neonates, in one of the above ways. To test the hypothesis that aromatization of androgen plays a regulatory role in proliferation of Sertoli cells, the aromatase inhibitor Triene will be implanted subcutaneously in neonates. The immediate and long-term effects of this treatment on Sertoli cell proliferation will then be determined. Studies will be carried out both in vivo, on intact rats, and in vitro, on cells isolated from neonates and on testicular explants from fetuses.